Tag Archives: George C. Schatz

Caption: Novel nanolaser leverages the same color-changing mechanism that a chameleon uses to camouflage its skin. Credit: Egor Kamelev Courtesy: Northwestern University

I wish there was some detail included about how those colo(u)rs were achieved in that photograph. Strangely, Northwestern University (Chicago, Illinois, US) is more interested in describing the technology that chameleons have inspired. A June 20, 2018 news item on ScienceDaily announces the research,

As a chameleon shifts its color from turquoise to pink to orange to green, nature’s design principles are at play. Complex nano-mechanics are quietly and effortlessly working to camouflage the lizard’s skin to match its environment.

Inspired by nature, a Northwestern University team has developed a novel nanolaser that changes colors using the same mechanism as chameleons. The work could open the door for advances in flexible optical displays in smartphones and televisions, wearable photonic devices and ultra-sensitive sensors that measure strain.

“Chameleons can easily change their colors by controlling the spacing among the nanocrystals on their skin, which determines the color we observe,” said Teri W. Odom, Charles E. and Emma H. Morrison Professor of Chemistry in Northwestern’s Weinberg College of Arts and Sciences. “This coloring based on surface structure is chemically stable and robust.”

The same way a chameleon controls the spacing of nanocrystals on its skin, the Northwestern team’s laser exploits periodic arrays of metal nanoparticles on a stretchable, polymer matrix. As the matrix either stretches to pull the nanoparticles farther apart or contracts to push them closer together, the wavelength emitted from the laser changes wavelength, which also changes its color.

“Hence, by stretching and releasing the elastomer substrate, we could select the emission color at will,” Odom said.

The resulting laser is robust, tunable, reversible and has a high sensitivity to strain. These properties are critical for applications in responsive optical displays, on-chip photonic circuits and multiplexed optical communication.

This research goes beyond the binary (0 or 1) and to an analog state that resembles quantum states. Fascinating, yes? An Oct. 10, 2016 news item on phys.org tells more,

Rice University scientists have discovered how to subtly change the interior structure of semi-hollow nanorods in a way that alters how they interact with light, and because the changes are reversible, the method could form the basis of a nanoscale switch with enormous potential.

“It’s not 0-1, it’s 1-2-3-4-5-6-7-8-9-10,” said Rice materials scientist Emilie Ringe, lead scientist on the project, which is detailed in the American Chemical Society journal Nano Letters. “You can differentiate between multiple plasmonic states in a single particle. That gives you a kind of analog version of quantum states, but on a larger, more accessible scale.”

Ringe and colleagues used an electron beam to move silver from one location to another inside gold-and-silver nanoparticles, something like a nanoscale Etch A Sketch. The result is a reconfigurable optical switch that may form the basis for a new type of multiple-state computer memory, sensor or catalyst.

At about 200 nanometers long, 500 of the metal rods placed end-to-end would span the width of a human hair. However, they are large in comparison with modern integrated circuits. Their multistate capabilities make them more like reprogrammable bar codes than simple memory bits, she said.

“No one has been able to reversibly change the shape of a single particle with the level of control we have, so we’re really excited about this,” Ringe said.

Altering a nanoparticle’s internal structure also alters its external plasmonic response. Plasmons are the electrical ripples that propagate across the surface of metallic materials when excited by light, and their oscillations can be easily read with a spectrometer — or even the human eye — as they interact with visible light.

The Rice researchers found they could reconfigure nanoparticle cores with pinpoint precision. That means memories made of nanorods need not be merely on-off, Ringe said, because a particle can be programmed to emit many distinct plasmonic patterns.

The discovery came about when Ringe and her team, which manages Rice’s advanced electron microscopy lab, were asked by her colleague and co-author Denis Boudreau, a professor at Laval University in Quebec, to characterize hollow nanorods made primarily of gold but containing silver.

“Most nanoshells are leaky,” Ringe said. “They have pinholes. But we realized these nanorods were defect-free and contained pockets of water that were trapped inside when the particles were synthesized. We thought: We have something here.”

Ringe and the study’s lead author, Rice research scientist Sadegh Yazdi, quickly realized how they might manipulate the water. “Obviously, it’s difficult to do chemistry there, because you can’t put molecules into a sealed nanoshell. But we could put electrons in,” she said.

Focusing a subnanometer electron beam on the interior cavity split the water and inserted solvated electrons – free electrons that can exist in a solution. “The electrons reacted directly with silver ions in the water, drawing them to the beam to form silver,” Ringe said. The now-silver-poor liquid moved away from the beam, and its silver ions were replenished by a reaction of water-splitting byproducts with the solid silver in other parts of the rod.

“We actually were moving silver in the solution, reconfiguring it,” she said. “Because it’s a closed system, we weren’t losing anything and we weren’t gaining anything. We were just moving it around, and could do so as many times as we wished.”

The researchers were then able to map the plasmon-induced near-field properties without disturbing the internal structure — and that’s when they realized the implications of their discovery.

“We made different shapes inside the nanorods, and because we specialize in plasmonics, we mapped the plasmons and it turned out to have a very nice effect,” Ringe said. “We basically saw different electric-field distributions at different energies for different shapes.” Numerical results provided by collaborators Nicolas Large of the University of Texas at San Antonio and George Schatz of Northwestern University helped explain the origin of the modes and how the presence of a water-filled pocket created a multitude of plasmons, she said.

The next challenge is to test nanoshells of other shapes and sizes, and to see if there are other ways to activate their switching potentials. Ringe suspects electron beams may remain the best and perhaps only way to catalyze reactions inside particles, and she is hopeful.

“Using an electron beam is actually not as technologically irrelevant as you might think,” she said. “Electron beams are very easy to generate. And yes, things need to be in vacuum, but other than that, people have generated electron beams for nearly 100 years. I’m sure 40 years ago people were saying, ‘You’re going to put a laser in a disk reader? That’s crazy!’ But they managed to do it.

“I don’t think it’s unfeasible to miniaturize electron-beam technology. Humans are good at moving electrons and electricity around. We figured that out a long time ago,” Ringe said.

The research should trigger the imaginations of scientists working to create nanoscale machines and processes, she said.

“This is a reconfigurable unit that you can access with light,” she said. “Reading something with light is much faster than reading with electrons, so I think this is going to get attention from people who think about dynamic systems and people who think about how to go beyond current nanotechnology. This really opens up a new field.”

According to an April 24, 2015 news item on Nanowerk, there has been a big discovery at Northwestern University (located in Chicago, Illinois, US),

Northwestern University scientists have developed the first liquid nanoscale laser. And it’s tunable in real time, meaning you can quickly and simply produce different colors, a unique and useful feature. The laser technology could lead to practical applications, such as a new form of a “lab on a chip” for medical diagnostics.

To understand the concept, imagine a laser pointer whose color can be changed simply by changing the liquid inside it, instead of needing a different laser pointer for every desired color.

In addition to changing color in real time, the liquid nanolaser has additional advantages over other nanolasers: it is simple to make, inexpensive to produce and operates at room temperature.

Nanoscopic lasers — first demonstrated in 2009 — are only found in research labs today. They are, however, of great interest for advances in technology and for military applications.

“Our study allows us to think about new laser designs and what could be possible if they could actually be made,” said Teri W. Odom, who led the research. “My lab likes to go after new materials, new structures and new ways of putting them together to achieve things not yet imagined. We believe this work represents a conceptual and practical engineering advance for on-demand, reversible control of light from nanoscopic sources.”

…

The liquid nanolaser in this study is not a laser pointer but a laser device on a chip, Odom explained. The laser’s color can be changed in real time when the liquid dye in the microfluidic channel above the laser’s cavity is changed.

The laser’s cavity is made up of an array of reflective gold nanoparticles, where the light is concentrated around each nanoparticle and then amplified. (In contrast to conventional laser cavities, no mirrors are required for the light to bounce back and forth.) Notably, as the laser color is tuned, the nanoparticle cavity stays fixed and does not change; only the liquid gain around the nanoparticles changes.

The main advantages of very small lasers are:

• They can be used as on-chip light sources for optoelectronic integrated circuits;

• They can be used in optical data storage and lithography;

• They can operate reliably at one wavelength; and

• They should be able to operate much faster than conventional lasers because they are made from metals.

Some technical background

Plasmon lasers are promising nanoscale coherent sources of optical fields because they support ultra-small sizes and show ultra-fast dynamics. Although plasmon lasers have been demonstrated at different spectral ranges, from the ultraviolet to near-infrared, a systematic approach to manipulate the lasing emission wavelength in real time has not been possible.

The main limitation is that only solid gain materials have been used in previous work on plasmon nanolasers; hence, fixed wavelengths were shown because solid materials cannot easily be modified. Odom’s research team has found a way to integrate liquid gain materials with gold nanoparticle arrays to achieve nanoscale plasmon lasing that can be tuned dynamical, reversibly and in real time.

The use of liquid gain materials has two significant benefits:

• The organic dye molecules can be readily dissolved in solvents with different refractive indices. Thus, the dielectric environment around the nanoparticle arrays can be tuned, which also tunes the lasing wavelength.

• The liquid form of gain materials enables the fluid to be manipulated within a microfluidic channel. Thus, dynamic tuning of the lasing emission is possible simply by flowing liquid with different refractive indices. Moreover, as an added benefit of the liquid environment, the lasing-on-chip devices can show long-term stability because the gain molecules can be constantly refreshed.

These nanoscale lasers can be mass-produced with emission wavelengths over the entire gain bandwidth of the dye. Thus, the same fixed nanocavity structure (the same gold nanoparticle array) can exhibit lasing wavelengths that can be tuned over 50 nanometers, from 860 to 910 nanometers, simply by changing the solvent the dye is dissolved in.